Hua Tuo didn't stop his disciple from trying new ways to find an answer after his hint. In fact, his job as a mentor was done. The rest was up to Zero to find his answer and learn by trial and error.
Bob and Mii didn't interfere with how Zero's wanted to test out but they did inform Hua Tuo when the teenager had gone too far. The lungfish experiment robbed Zero of his hope when he found out that paying with insurance didn't work with the Aggression Falls. It was as if the waterfall didn't give a handicap to even marine life. Even as a fish with the ability to breathe in both air and water, Zero stood no chance. He still felt as if the water was trying to drown him in an awful manner that made his brain confused. The teenager spent the next hour recuperating and learning how to breathe correctly before he challenged the waterfall again.
